1. Field of the Invention
The invention is related to satellite communications and, more particularly, to geosynchronous satellites and communications based upon them.
2. Description of the Related Art
FIG. 1 illustrates a communications system employing three repeater stations which when spaced 120 degrees apart in the correct orbit could give television and microwave coverage to the entire planet. Using such a system allows a user to transmit a signal (e.g., video, voice data, etc.) from one ground station A to a "repeater" satellite B to another ground station C within the repeater satellite's field of view. For communications outside a particular satellite's field of view, the ground station A transmits to the satellite B which relays the signal to the ground station C. The ground station C is in the field of view of both satellite B and another satellite D. Ground station C relays the signal to satellite D, which transmits it to a ground station E within its field of view. Each "uplink", including amplifiers, antennas, and modulators at the ground station and receivers aboard the satellite, typically operate within one frequency band while the downlink operates within another. Transponders aboard the satellites typically bandpass filter an incoming signal from a receiving antenna and amplify the resultant signal using a low noise amplifier. The amplified signal is then down-converted by the transponder so that the signal occupies a different frequency band. This separation into transmitted and received signal bands prevents transmitted signals from interfering with received signals. After down-conversion, the transponder bandpass filters the down-converted signal and amplifies the resultant signal using a power amplifier such as a traveling wave tube amplifier.
Although the "ground-hopping" technique of FIG. 1 has the appeal of simplicity, it is not without its problems. The volume of communications traffic has increased markedly since the first operational synchronous satellite, Syncom II, was launched Jul. 26, 1963 and transmitted communications between a U.S. Army crew in Lakehurst N.J. and a U.S. Navy crew aboard the Kingsport, at harbor in Lagos, Nigeria. Not only has the volume of traffic increased for existing applications such as voice traffic, new applications have evolved which place a huge demand upon satellite channel capacity. "Superstations" originating in Atlanta and Chicago reach homes worldwide, as do motion pictures that are distributed directly from satellites to homes. The volume of data communications over satellite links has also expanded rapidly over the past twenty years.
Increased traffic volume led to revisions of the basic model illustrated in FIG. 1. For example, each satellite in FIG. 1 broadcasts over the more than 120 degrees of the Earth's surface within its field of view. Every terrestrial station receives the same signals at the same frequency. However vast the frequency spectrum may seem, it is limited and if, for example, one wishes to transmit one signal to one ground station using a given frequency channel, a different signal would be forced to use a different channel. The number of messages transmitted would therefore be limited, in a first order analysis, to the number of channels available within the frequency bands allotted to the satellite system. One method of expanding the number of messages that may be sent is to use narrow beam antennas to direct messages to smaller geographic areas. With this approach, frequency channels may be re-used in different geographic locations within the view of each satellite. This approach has been employed to expand the communications capacity of satellite systems.
The basic operational systems of a conventional communications satellite, including crosslinks, are illustrated in the block diagram of FIG. 2A. Communications satellites and their interfaces are known in the art; a more detailed description of the filtering, multiplexing, modulation, switching etc. may be found, for example, in, James Martin, Telecommunications and the Computer, Prentice Hall, Englewood Cliffs, N.J., 1976, pages 280-301. In the diagram of FIG. 2A, a ground link interface 13 provides an uplink, a downlink or both, between the satellite and a ground station such as ground station A illustrated if FIG. 1. Similarly, crosslink interfaces 15 and 17 provide for communications between neighboring satellites. The switching block 19 controls the flow of communications from one interface to another.
FIG. 2B illustrates an interface in greater detail. This interface could be a crosslink interface such as 15 or 17 of FIG. 2A or a downlink interface such as 13. The illustrated interface is employed in satellites which are essentially "bent pipe" relays, i.e., the satellites perform no processing on received signals and simply relay them to another satellite or ground station. A receiving antenna 21 receives signals which have been multiplexed into a specified frequency band and relays the signals to a receiver 23. From the receiver, the signals are sent to a filter bank 25 which separates the various signals and their carriers. These are sent to the switching block 19 for routing to another interface. Signals coming from the switching block 19 are connected to a transmitter 27 which sends the signals to a transmitting antenna 29.
An interface used in processing satellites, i.e. those which demodulate received signals and perform signal conditioning, compression, or regeneration, etc., is illustrated in FIG. 2C. A receiving antenna 31 receives transmissions from another satellite or a ground station and passes the signal to a receiver 33. The receiver then passes the signal to a filter bank 35 which separates the received transmission into individual frequency bands. Demodulators 37 demodulate the signals and pass them to a processing subsystem 39 which may perform signal conditioning, regeneration, etc. upon the recovered signals. From the processing subsystem 39, the signals are passed to a switching block, such as block 19 shown in FIG. 2A, which routes the signals as described in relation to that FIG. On the transmitting side, signals received from a switching block for transmission are routed to a modulator 41 where they are combined with carrier frequencies for transmission. The modulated signals are passed to a multiplexer 43 which combines the modulated signals and passes the resultant signal to a transmitter 45 that employs an antenna 47 to transmit the signals to a ground station or to another satellite. One satellite may have several narrow beam antennas directed toward different locations on the Earth's surface (with accompanying transmitters, modulators and multiplexers, where appropriate).
The use of narrow beam antennas in combination with ground hopping has expanded the capacity of satellite systems and constitutes the standard model for satellite communications systems. But capacity-related problems remain. The most desirable frequency bands are a 500 MHz band centered at 6 Ghz for satellite uplinks and a 500 MHZ band centered at 4 Ghz for downlinks. In these bands the equipment is relatively inexpensive, cosmic noise is small, and losses due to ionospheric scintillation and rainfall are relatively insignificant. However, terrestrial microwave links are already assigned to operate in these bands. To accommodate growing communications traffic, additional, less desirable, bands have been assigned; communications in the K-band employ 14 Ghz on the uplink and 12 Ghz on the downlink and direct-to-the-home television broadcasting uses a 17 Ghz uplink and a 12 Ghz downlink.
Even with this capacity expansion, satellite systems are once again confronting capacity limitations. The downlinks used to relay signals from satellites to ground stations often create bottlenecks, in part because these links must share their frequency band with terrestrial microwave transmission systems that can cause interference and in part because communications traffic from many parts of the world are essentially "funneled" through the downlinks, even though the ground station at the receiving end of the downlink may not be the ultimate target of the communications. Additionally, each "hop" entails a trip of approximately 70,000 km with a concomitant signal delay of approximately 240 ms. Although this delay may not be significant in some signal applications, interactive applications such as voice cannot practically accommodate more than a few such delays. Atmospheric distortions degrade signals as they hop from satellite to ground and on to another satellite. Each transit of a satellite introduces further signal degradation in the form of phase noise, distortions and intermodulation products generated by the satellite's transponders. Depending upon the application and the initial signal quality, there may be a need for signal regeneration. Satellites which perform this regeneration are sometimes referred to as "processing" satellites. The additional functionality of processing satellites comes at a price and it is generally desirable to limit the number of processing satellites within a satellite system.